Magnesium-chloride-supported high-mileage catalysts for olefin polymerization. III. Electron paramagnetic resonance studies

Electron paramagnetic resonance (EPR) was used to study a MgCl₂-supported, high-mileage olefin polymerization catalyst. Anhydrous Toho MgCl₂ was the starting material. Treatment with HCl at an elevated temperature, ethyl benzoate by ball-milling, p-cresol, AlEt₃, and TiCl₄produced a catalyst that contained a single EPR observable Ti⁺³ species A, which was strongly attached to the catalyst surface, had a D_3h symmetry, and no other Ti⁺³ ion in an immediately adjacent site. Species A constitutes only 20% of all the trivalent titaniums; the remainder is EPR-silent and may be attributed to those Ti⁺³ ions that have adjacent sites occupied by one or more Ti⁺³ ions. Activation with preformed AlEt₃/methyl-p-toluate complexes produced a single Ti⁺³ species (C) with rhombic symmetry and displaying ²⁷Al superhyperfin splitting which has attributes for a stereospecific active site. This species is unstable under polymerization conditions and is transformed to another species with axial symmetry and solubilization. Both processes could lead to catalyst deactivation and loss of stereospecificity. Catalysts activated by AlEt₃ and methyl-p-toluate separately in various sequential orders produced a multitude of EPR-observable Ti⁺³ species with varying degrees of motional freedom deemed detrimental to stereospecific polymerization of α-olefins.     

Superactive and stereospecific catalysts. III. Definitive identification of active sites by electron paramagnetic resonance

Superactive Ziegler–Natta catalysts have been prepared from a soluble MgCl₂·2-ethyl hexanol adduct in the presence of organic esters through reactions with TiCl₄ and activated with AlEt₃/phenyltriethoxy-silane. Electron paramagnetic spectra (EPR) were used to elucidate the nature and amount of those Ti⁺³ ions not bridged to another Ti⁺³ ion; the chlorine bridged Ti⁺³ ions are EPR silent. The EPR spectra were attributed to two rhombic Ti⁺³ sites with principal values for the g-tensors (1.967, 1.949, 1.915; and 1.979, 1.935, 1.887). The total amount of the EPR species, obtained by double integration of the EPR spectra, is in close agreement with the concentration of isospecific catalytic sites determined by radiotagging. This suggests that the nonspecific sites are EPR silent. When o-phthalic ester was present during the catalyst synthesis, there appears an EPR signal at the free electron g-value. This signal was attributed to a Ti⁺³ phthalate species with resonance stabilization and spin delocalization; it is absent in the catalysts made in the presence of monoesters such as ethyl benzoate. The effects of monomer, O₂, H₂O, and I₂ on the EPR spectra were investigated. The changes in the EPR spectral intensity and the total Ti⁺³ ions, the latter determined by redox titrations during a polymerization or catalyst aging, are described. The results were extensively compared with those observed for supported Ziegler–Natta catalyst prepared with crystalline MgCl₂.     

Magnesium chloride supported high-mileage catalysts for olefin polymerization. I. Chemical composition and oxidation states of titanium

The chemical composition of a MgCl₂-supported, high-mileage catalyst has been determined at every stage of its preparation. Ball milling of MgCl₂ with ethyl benzoate (EB) resulted in the incorporation of 95% of the EB present to give MgCl₂·EB_0.15. A mild reaction with a half-mole equivalent of p-cresol (PC) at 50°C for 1 h resulted in near quantitative retention of p-cresol by the support. The composition is now approximately MgCl₂·EB_0.15P?_0.5. Addition of an amount of AlEt₃ corresponding to half-mole equivalent of p-cresol liberated one mole of ethane per mole of p-cresol, thus signaling quantitative reaction between the two components. The support contains on the average one ethyl group per Al. Further reaction with TiCl₄ resulted in the incorporation of titanium of approximately 8, 38, and 54% in the oxidation states of +2, +3, and +4, respectively. The ratio of Al to Ti in the catalyst lies in the range of 0.5–1.0. Only 19% of all the Ti⁺³ species in the catalyst can be observed by electron paramagnetic resonance (EPR); these are attributable to isolated Ti⁺³ complexes. The remaining EPR silent Ti⁺³ species are believed to be bridged to another Ti⁺³ by Cl ligands. The total Cl content is equal to the sum of 2 × Mg + 3 × Al + 3.5 × Ti. Most of the p-cresol moiety apparently disappeared from the support, leaving much of ethyl benzoate in the catalyst. Activation with AlEt₃/methyl-p-toluate complex reduces 90% of the Ti⁺⁴ in the catalyst to lower oxidation states. The ester apparently moderates the alkylating power of AlEt₃ to avoid excessive formation of divalent titanium sites. There appears to be a constant fraction of 1/4–1/5 of the titanium which is isolated and the remainder is in bridged clusters independent of the oxidation states of titanium.     

Supported titanium‐magnesium catalysts for ethylene polymerization: A comparative study of catalysts containing isolated and clustered titanium ions in different oxidation states

Supported titanium–magnesium catalysts (TMC) comprising isolated and clustered titanium ions in different oxidation states, which are obtained using titanium compounds of different composition (TiCl₄, TiCl₃?nDBE (DBE – dibutyl ether), [η⁶–BenzeneTiAl₂Cl₈]), were synthesized and tested in ethylene polymerization. The state of titanium ions was studied by the ESR method both for the procatalysts and after their interaction with triisobutilaluminum. For identification of ESR‐silent Ti³⁺ ions and Ti²⁺ ions, special procedures of additional catalyst treatment with pyridine, water, and chloropentafluorobenzene were used to obtain Ti³⁺ ions that are observable in ESR spectra. In distinction to numerous earlier works performed with the TiCl₄/MgCl₂ catalyst comprising after the interaction with AlR₃ the Ti³⁺ surface compounds both as isolated ions and clusters (ESR‐silent), this work considers the [η⁶–BenzeneTiAl₂Cl₈]/MgCl₂ catalyst (TMC‐3) comprising mainly the isolated Ti²⁺ ions and a new catalyst TMC‐4 obtained by treating the TMC‐3 with chloropentafluorobenzene. This catalyst comprises only the isolated Ti³⁺ ions both before and after the interaction with triisobutylaluminum. It was shown that in spite of sharp distinctions between the catalysts under consideration concerning titanium oxidation state and the ratio of isolated Ti³⁺ ions to clustered ones, all these catalysts produce polyethylenes with similar molecular weights and molecular‐weight distributions. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6362–6372, 2009     

High activity magnesium chloride supported catalysts for olefin polymerization. XXIX. Molecular basis of hydrogen activation of magnesium chloride supported Ziegler–Natta catalysts

Hydrogen (pH₂ = 72 torr) increases the rate of propylene polymerization by a MgCl₂/ethyl benzoate/p-cresol/AlEt₃/TiCl₄-AlEt₃/methyl-p-toluate catalyst (CW-catalyst) by two-to three-fold which corresponds closely with the increase in the number of active sites as counted by radiolabeling with tritiated methanol. The oxidation states of titanium in decene polymerizations by the CW-catalyst were determined as a function of time of polymerization (t_p). In the absence of H₂, all [Ti⁺ⁿ] for n = 2, 3, 4 remain constant during a batch polymerization. In the presence of H₂ and within 5 min of t_p, [Ti⁺²] decreases by an amount, corresponding to 15% of the total titanium and [Ti⁺³] increases by the same amount, while [Ti⁺⁴] is not changed. Therefore, three-fourths of the H₂ activation result from oxidative addition processes. The remaining one-fourth of the H₂ activation may be attributed to the activation of previously deactivated Ti⁺³ ions by hydrogenolysis. Monomer converts some of the EPR silent Ti⁺³ sites to EPR observable species resulting in their activation.     

Probing the Electronic Structure and Photoactivation Process of Nitrogen‐Doped TiO2 Using DRS,PL, and EPR

The electronic structure and photoactivation process in N‐doped TiO₂ is investigated. Diffuse reflectance spectroscopy (DRS), photoluminescence (PL), and electron paramagnetic resonance (EPR) are employed to monitor the change of optical absorption ability and the formation of N species and defects in the heat‐ and photoinduced N‐doped TiO₂ catalyst. Under thermal treatment below 573 K in vacuum, no nitrogen dopant is removed from the doped samples but oxygen vacancies and Ti³⁺ states are formed to enhance the optical absorption in the visible‐light region, especially at wavelengths above 500 nm with increasing temperature. In the photoactivation processes of N‐doped TiO₂, the DRS absorption and PL emission in the visible spectral region of 450–700 nm increase with prolonged irradiation time. The EPR results reveal that paramagnetic nitrogen species (N_s^.), oxygen vacancies with one electron (V_o^.), and Ti³⁺ ions are produced with light irradiation and the intensity of N_s^. species is dependent on the excitation light wavelength and power. The combined characterization results confirm that the energy level of doped N species is localized above the valence band of TiO₂ corresponding to the main absorption band at 410 nm of N‐doped TiO₂, but oxygen vacancies and Ti³⁺ states as defects contribute to the visible‐light absorption above 500 nm in the overall absorption of the doped samples. Thus, a detailed picture of the electronic structure of N‐doped TiO₂ is proposed and discussed. On the other hand, the transfer of charge carriers between nitrogen species and defects is reversible on the catalyst surface. The presence of oxygen‐vacancy‐related defects leads to quenching of paramagnetic N_s^. species but they stabilize the active nitrogen species N_s^?.     

Propylene Polymerization Performance of Isolated and Aggregated Ti Species Studied Using a Well‐Designed TiCl3/MgCl2 Ziegler‐Natta Model Catalyst

In propylene polymerization with MgCl₂‐supported Ziegler‐Natta catalysts, it is known that the reduction of TiCl₄ with alkylaluminum generates Ti³⁺ active species, and at the same time, leads to the growth of TiCl_x aggregates. In this study, the aggregation states of the Ti species were controlled by altering the Ti content in a TiCl₃/MgCl₂ model catalyst prepared from a TiCl₃ · 3C₅H₅N complex. It is discovered that all the Ti species become isolated mononuclear with a highly aspecific feature below 0.1 wt.‐% of the Ti content, and that the isolated aspecific Ti species are more efficiently converted into highly isospecific ones by the addition of donors than active sites in aggregated Ti species.

     

Sulfur and Ti3+ co‐Doping of TiO2 Nanotubes Enhance Photocatalytic H2 Evolution Without the Use of Any co‐catalyst

TiO₂ nanotubes were successfully co‐doped with sulfur and Ti³⁺ states using a facile annealing treatment in H₂/H₂S gas mixture. The obtained nanotubes were investigated for their photocatalytic performance and characterized by SEM, XRD, XPS, EPR, IPCE, IMPS and Mott‐Schottky measurements. The synthesized co‐doped TiO₂ nanotubes show an enhanced photocatalytic hydrogen production rate compared to tubes that were treated only in pure H₂ or H₂S atmosphere—this without the presence of any co‐catalyst. It was found that sulfur in co‐doped TiO₂ exists in the form of S^2? and a small quantity of S⁴⁺/S⁶⁺, which leads to a narrowing of the band gap. However, the enhanced absorption of light in the visible range is not the key reason for the improved photocatalytic performance. We ascribe the enhanced photocatalytic activity to a synergetic effect of S mid‐gap states and disordered Ti³⁺ defects that facilitate photo generated electron transfer.     

Stable Co‐Catalyst‐Free Photocatalytic H2 Evolution From Oxidized Titanium Nitride Nanopowders

A simple strategy is used to thermally oxidize TiN nanopowder (～20 nm) to an anatase phase of a TiO₂:Ti³⁺:N compound. In contrast to the rutile phase of such a compound, this photocatalyst provides activity for hydrogen evolution under AM1.5 conditions, without the use of any noble metal co‐catalyst. Moreover the photocatalyst is active and stable over extended periods of time (tested for 4 months). Importantly, to achieve successful conversion to the active anatase polymorph, sufficiently small starting particles of TiN are needed. The key factor for catalysis is the stabilization of the co‐catalytically active Ti³⁺ species against oxidation by nitrogen present in the starting material.     

EPR Studies of Photoreduction of Ni/TiO2 Catalysts

Irradiations of Ni/TiO₂ catalyst by UV in hydrogen at 77 K produced not only Ni⁺ ions on the catalyst surface, but also Ni³⁺ and Ti³⁺ species in bulk or near the interface between nickel and titania. These photo-generated species were detected and characterized by low temperature electron paramagnetic resonance (EPR) spectroscopy. Relative spin concentrations of the photogenerated paramagnetic species (Niⁿ⁺ and Ti³⁺) varied with the nickel content in titania. A high nickel content in the sample resulted in a high peak intensity ratio of Niⁿ⁺ to Ti³⁺. It was found that the photoinduced self-redox reaction of Ni²⁺ ions to form Ni⁺ and Ni³⁺ ions has a priority over the photoreduction of Ti⁴⁺ to Ti³⁺ ions. The characteristic EPR spectrum of the Ni³⁺ (3d⁷) ions with g₁ = 2.268, g₂ = 2.237, and g₃ = 2.045 indicates that the Ni³⁺ ions are most likely located in the substitutional sites of TiO₂, possibly near the surface rutile phase. The Ni⁺ species (3d⁹) with g₄ = 2.130 and g₁ = 2.063 are on the surface of TiO₂. Both Ni⁺ and Ni³⁺ ions are quite stable in hydrogen. The Ni³⁺ ions seem to be responsible for anchoring the nickel ions onto titania and stablizing the Ni⁺ species on the surface. The Ni⁺ ions are thus free from oxygen poisoning and still show a high activity toward olefin oligomerization.     

State of titanium in supported titanium-magnesium catalysts for propylene polymerization

The oxidation state of titanium and the coordination state of Ti³⁺ ions in TiCl₄/D₁/MgCl₂ (D₁ is a phthalate) supported titanium-magnesium catalysts (TMCs) after the interaction with an AlEt₃/D₂ cocatalyst (D₂ is propyltrimethoxysilane or dicyclopentyldimethoxysilane) were studied by chemical analysis and EPR spectroscopy. Different oxidation state distributions of titanium ions were observed in the activated catalyst and mother liquor: Ti³⁺ and Ti²⁺ ions were predominant in the activated catalyst and mother liquor, respectively. The effects of interaction conditions (reaction temperature and time and Al/Ti and D₂/Ti molar ratios) of TMCs with the cocatalyst on the state of titanium in activated samples were studied. The interaction of TMCs with the cocatalyst decreased the titanium content and caused the appearance of aluminum in the activated sample, which was most clearly pronounced at a temperature of 25°C and occurred within the first 10 min of treatment. An increase in the temperature to 70°C and an increase in the interaction time to 60 min only slightly affected the concentrations of titanium and aluminum. The presence of D₂ as a cocatalyst constituent facilitated the removal of titanium compounds and restricted the adsorption of aluminum compounds on the catalyst surface. The main fraction of titanium consisted of Ti³⁺ ions (62–89%), and the rest was Ti⁴⁺ ions (22–35%) under mild interaction conditions (25°C; Si/Ti = 25) or Ti⁴⁺ (0–21%) and Ti²⁺ (9–21%) ions under more severe conditions (50 or 70°C; Si/Ti from 0 to 5). According to EPR-spectroscopic data, at D₂/Ti from 1 to 5, Ti³⁺ ions mainly occurred as associates, whereas they occurred as isolated ions at D₂/Ti = 25. The initial and activated catalysts were similar in activity in the reaction of propylene polymerization, and titanium compounds, which were removed from the catalyst upon interaction with AlEt₃/D₂, were inactive in this process.     

Crystalline titanium dichloride—an active catalyst in ethylene polymerization. I. Catalyst activation

Crystalline titanium dichloride, in the absence of organometallic cocatalyst, is a very poor catalyst for the polymerization of ethylene. It is transformed into a very active catalyst through mechanical activation (ball-milling). This catalyst is active in the absence not only of organometallic cocatalysts, but also metals and compounds (such as aluminium and AlCl₃) capable of forming organometallic compounds in situ (i.e., with ethylene, before polymerization starts). Ball-milling causes not only the expected increase in surface area but also disproportionation of Ti⁺⁺ to Ti⁺⁺⁺ and metallic titanium, as well as a crystal phase change to a structure not previously identified with those of TiCl₂ or TiCl₃. Catalyst activity (polymerization rate) is shown to be proportional to surface area and a direct function of Ti⁺⁺ content of the catalyst; an empirical equation relates catalyst activity to surface area and to Ti⁺⁺ lost through disproportionation. Titanium trichloride was found to be inactive in the absence of organometallic cocatalyst, even after ball-milling. The difference in structure of the catalytically active species in the conventional Ziegler (organometallic cocatalyst) and in the titanium dichloride catalyst are discussed. The mechanism of polymerization is compared with that of the supported (CrO₃ on SiO₂/Al₂O₃ and MoO₃ on Al₂O₃) catalyst systems.     

Carbon monoxide and oxygen interaction with Ru/MgF<Subscript>2</Subscript> catalyst: IR and EPR studies

Electron paramagnetic resonance (EPR) and infrared (IR) spectroscopy were used to study the formation of ruthenium and adsorbed species appearing on the catalyst upon the adsorption of CO and O₂ on 1.37 wt% Ru/MgF₂ catalysts derived from Ru₃(CO)₁₂. The presence of Ru ^x+ sites in spite of a reductive H₂ treatment at 673 K was observed by EPR and IR spectroscopy beside metallic Ru⁰ species. Both IR and EPR results provided clear evidence for the interaction between surface ruthenium and probe molecules. The IR spectra recorded after admission of CO showed a band at approx. 2000 cm⁻¹, due to linearly adsorbed CO on Ru⁰/MgF₂ and two bands at higher frequencies (approx. 2140 and approx. 2070 cm⁻¹), related to CO on oxidized Ru ⁿ⁺ species, e.g., to Ru(CO)₃ complex with Ru in the 1+ and/or 2+ state of oxidation and Ru(CO)₂ with Ru in the 3+ and/or 4+ state of oxidation. A weak anisotropic EPR signal with g _‖ = 2.017 and g _⊥ = 2.003 is due to O ₂ ⁻ radicals and a formation of Ru⁴⁺-O ₂ ⁻ complex is postulated. The Ru³⁺ appears to oxidize to Ru⁴⁺ and the resulting dioxygen anion is coordinated to the ruthenium. The strong, isotropic EPR signal at g ₀ = 2.003 detected upon admission of CO is attributed to CO radical anion rather than to any ruthenium carbonyl complexes.     

Carbonic Anhydrase‐Mimetic Bolaamphiphile Self‐Assembly for CO2 Hydration and Sequestration

A biomimetic catalyst was prepared through the self‐assembly of a bolaamphiphilic molecule with histidine moieties for the sequestration of carbon dioxide. The histidyl bolaamphiphilic molecule bis(N‐α‐amidohistidine)‐1,7‐heptane dicarboxylate has been synthesized and self‐assembled to produce analogues of the active sites of carbonic anhydrase (CA) after association with Zn²⁺ ions. Spectroscopic analysis demonstrated the coordination of the Zn²⁺ ions with histidine imidazole moieties, which is the core conformation of CA active sites. The Zn‐associated self‐assembly worked as a CA‐mimetic catalyst that shows catalytic activity for CO₂ hydration. Evaluation of the kinetics of using para‐nitrophenylacetate revealed that the kinetic parameters of the CA‐mimetic catalyst were maximized at the optimal Zn concentration and that excess Zn ions resulted in deteriorated catalytic activity. The performance of the CA‐mimetic catalyst was enhanced by changing the pH value and temperature of the reaction, which implies that the hydrolysis of the substrate is the rate‐determining step. The catalyst‐assisted sequestration of CO₂ was demonstrated by CaCO₃ precipitation upon the addition of Ca²⁺ ions. This study offers an easy way to prepare enzyme analogues for CO₂ sequestration through the self‐assembly of bolaamphiphile molecules with designer biochemical moieties.     

Size Selectivity of a Copper Metal–Organic Framework and Origin of Catalytic Activity in Epoxide Alcoholysis

{Cu(bpy)(H₂O)₂(BF₄)₂(bpy)} (Cu‐MOF; MOF=metal–organic framework; bpy=4,4′‐bipyridine), with a 3D‐interpenetrated structure and saturated Cu coordination sites in the framework, possesses unexpectedly high activity in the ring‐opening reaction of epoxides with MeOH, although the reaction rate drops remarkably with more bulky alcohols. This (apparent) size selection and the single Cu²⁺ sites in an identical environment of the crystalline matrix resemble zeolites. The real nature of active sites was investigated by attenuated total reflection infrared (ATR‐IR), Raman, EPR, and UV/Vis spectroscopies. Cu‐MOF has highly dynamic structural properties that respond to MeOH; its framework dimensions change from 3D to 2D by restructuring to a symmetric coordination of four bpy units to Cu. This interaction is accompanied by the partial dissolution of Cu‐MOF as multi‐Cu clusters, in which Cu²⁺ ions are connected with bpy ligands. Although both molecular and surface catalysis contribute to the high rate of alcoholysis, the soluble oligomeric species (Cu_mbpy_n) are far more active. Finally, addition of diethyl ether to the reaction mixture induces the reconstruction of dissolved and solid Cu‐MOF to the original framework structure, thereby allowing excellent recyclability of Cu‐MOF as an apparent heterogeneous catalyst. In contrast, the original Cu‐MOF structure is maintained upon contact with larger alcohols, such as iPrOH and tBuOH, thus leading to poor activity in epoxide ring opening.     

Highly efficient heterogeneous acylations of aromatic compounds with acid anhydrides and carboxylic acids by montmorillonite-enwrapped titanium as a solid acid catalyst

Montmorillonite-enwrapped titanium hydroxide species (Ti⁴⁺-mont) acted as a highly efficient heterogeneous acid catalyst for the acylation of aromatic compounds with acid anhydrides or carboxylic acids. The catalytic activity of the Ti⁴⁺-mont was higher than those of other acid catalysts such as zeolites, SO ₄ ²⁻ /ZrO₂ and p-toluenesulfonic acid. For example, the reaction of anisole with dodecanoic acid in the presence of the Ti⁴⁺-mont catalyst gave 1-(4-methoxyphenyl)-1-dodecanone in 97% yield. Furthermore, the Ti⁴⁺-mont catalyst was easily separated from the reaction mixture and was recyclable.     

Vacuum activation-induced Ti<Superscript>3+</Superscript> and carbon co-doped TiO<Subscript>2</Subscript> with enhanced solar light photo-catalytic activity

Ti³⁺ and carbon co-doped TiO₂ photocatalysts were prepared hydrothermally to introduce the carbon, and followed by simple vacuum activation to achieve the Ti³⁺ self-doping. The prepared co-doped photocatalysts were characterized by XRD, TEM, UV–Vis absorption spectra, EPR, and XPS. It was found that the co-doped TiO₂ has dispersed nanoparticles and a narrower band-gap compared with the un-doped TiO₂ and single-doped TiO₂. The experimental results displayed that the coke carbon generated on the surface of co-doped TiO₂ acts as a photosensitizer and has the photosensitization effect under solar light irradiation. Except for the carbon sensitization effect, the Ti³⁺ self-doping modification has a synergistic effect which is the reason for the effective photo-degradation of methyl orange under simulated solar light irradiation.     

Slow Magnetic Relaxation from Hard‐Axis Metal Ions in Tetranuclear Single‐Molecule Magnets

We report the synthesis of the novel heterometallic complex [Fe₃Cr(L)₂(dpm)₆]?Et₂O ( Fe₃CrPh ) (Hdpm=dipivaloylmethane, H₃L=2‐hydroxymethyl‐2‐phenylpropane‐1,3‐diol), obtained by replacing the central iron(III) atom by a chromium(III) ion in an Fe₄ propeller‐like single‐molecule magnet (SMM). Structural and analytical data, high‐frequency EPR (HF‐EPR) and magnetic studies indicate that the compound is a solid solution of chromium‐centred Fe₃Cr (S=6) and Fe₄ (S=5) species in an 84:16 ratio. Although SMM behaviour is retained, the |D| parameter is considerably reduced as compared with the corresponding tetra‐iron(III) propeller (D=?0.179 vs. ?0.418 cm^?1), and results in a lower energy barrier for magnetisation reversal (U_eff/k_B=7.0 vs. 15.6 K). The origin of magnetic anisotropy in Fe₃CrPh has been fully elucidated by preparing its Cr‐ and Fe‐doped Ga₄ analogues, which contain chromium(III) in the central position (c) and iron(III) in two magnetically distinct peripheral sites (p1 and p2). According to HF‐EPR spectra, the Cr and Fe dopants have hard‐axis anisotropies with D_c=0.470(5) cm^?1, E_c=0.029(1) cm^?1, D_p1=0.710(5) cm^?1, E_p1=0.077(3) cm^?1, D_p2=0.602(5) cm^?1, and E_p2=0.101(3) cm^?1. Inspection of projection coefficients shows that contributions from dipolar interactions and from the central chromium(III) ion cancel out almost exactly. As a consequence, the easy‐axis anisotropy of Fe₃CrPh is entirely due to the peripheral, hard‐axis‐type iron(III) ions, the anisotropy tensors of which are necessarily orthogonal to the threefold molecular axis. A similar contribution from peripheral ions is expected to rule the magnetic anisotropy in the tetra‐iron(III) complexes currently under investigation in the field of molecular spintronics.     

Oxidation of sulfides including DBT using a new vanadyl complex of a non‐innocent o‐aminophenol benzoxazole based ligand

Reaction of a non‐innocent o‐aminophenol benzoxazole based ligand HL^BAP with VOCl₃ afforded a vanadyl complex, VOL^BIS (SQ), in which SQ is a 2,4‐di‐tert‐butylsemiquinone produced from hydrolysis of HL^BAP. The crystal structure of VOL^BIS (SQ) exhibits an octahedral geometry with the VO²⁺ center coordinated by two nitrogen and one oxygen atoms of L^BAP and two oxygen atoms of SQ. Electrochemical studies showed quasi‐reversible metal‐centered reduction and ligand‐centered oxidation of complex. The magnetic moment of VOL^BIS (SQ) is consistent with the spin‐only value expected for S = 1/2 system. The neutral species of VOL^BIS (SQ) is EPR active, which is consistent with a paramagnetic electronic ground state (S = 1/2). This result is in accordance with the vanadyl (IV) moiety surrounded by tridentate iminobenzosemiquinonate anion radical (HL^BIS)^?‐ and benzosemiquinone ligand (SQ)^?. The theoretical calculations confirm the experimental results. Furthermore, we present the optimal conditions for maximum efficiency of sulfide oxidation for oxidative desulfurization with hydrogen peroxide and 6 times reusability of catalyst for sulfoxidation of dibenzothiophene.